1. Field of the Invention
The present invention relates to a toner for electrostatic image development and a process for preparing the same. More specifically, the present invention relates to a toner for electrostatic image development wherein properties such as chroma and transparency are improved and to a process for preparing the same.
2. Description of Related Art
Toners for use in electrophotography tend to have a reduced particle diameter in order to satisfy a recent demand for high definition images. Toners now being used are fine particle toners having in general an average particle diameter of about 5 to 15 μm. Typically, such fine particle toners are produced by the so-called crushing method comprising mechanically kneading a resin, a pigment and the like and then crushing the resultant kneaded product. The preparation of toners by the crushing method, however, is disadvantageous in that with reducing diameter of a toner produced, more complicated facility and procedure are required for the preparation of the toner and thus preparation costs of the toner increases. The preparation of toners by the crushing method is disadvantageous also in that toners prepared by the crushing method are unstable in chargeability because a pigment contained in the toner may be exposed at a particle surface. Mentioned as a toner preparation method that meets the demand for toner particle diameter reduction and that is more advantageous in productivity than the crushing method are polymerization methods such as a suspension polymerization method and an emulsion polymerization aggregation method. The polymerization methods, however, have a disadvantage that the resin is limited to an acrylic resin, and are inappropriate for preparation of color toners and for lower-temperature fixation and offset prevention of toners.
Toner preparation methods that have no limitation on the resin and that is more advantageous in productivity than the crushing method include an in-water drying method. The in-water drying method is advantageous for toner particle diameter reduction and, by the choice of pigment and resin, can also prevent the pigment from being exposed at a particle surface.
In recent years, there has been a demand for toners containing a pigment in an increased concentration, in view of a reduction of the amount of toners consumed. Such toners containing a pigment in an increased concentration suffer from transparency degradation and hue shift caused by an unreacted organic color pigment, which have not been greatly problematic with toners containing a pigment in a conventional concentration. Further, with increasing concentration of an organic pigment in a toner, toner particles come to stand closer to each other and become more liable to form aggregates. This makes it difficult to increase the density of a color of the toner solely by increasing the concentration of the organic pigment. Therefore, there is a limit to solutions of the above problems solely by the choice of organic pigment, resin and wax.
In general, pigments have merits of high light-resistance and high water resistance, while they have a demerit of lack of transparency.
Japanese Unexamined Patent Publication No. Hei 4(1992)-330462 proposes a color toner that has a combination of light-resistance, which is a feature of pigments, and clear hue and transparency, which are features of dyes. This toner, which contains a colorant obtained by reacting a reactive dye with a polymer having, on its side chain, an amino group and a hydroxyl group, can form an image having high transparency and high weather resistance. However, since the colorant is prepared in water using the dye and the highly hygroscopic polymer, the toner particles obtained are highly hygroscopic. This presents a problem that with increasing concentration of the colorant in the toner, the toner also becomes increasingly hygroscopic.
Japanese Unexamined Patent Publication No. Hei 11(1999)-231572 proposes a toner excellent in pigment dispersibility, transparency and color developability. This toner, which has high transparency, can be prepared by highly dispersing an organic or inorganic pigment in a binding resin with use of a synergist and a polymer dispersant. However, synergists resemble in skeleton to a pigment and have an aromatic ring and an auxochrome, and thus, in many cases, have a slight color. Further, as mentioned in Japanese Unexamined Patent Publication No. Hei 11(1999)-231572, synergists, which could possibly affect (decrease) the electric charge of a toner, cannot be used in a great amount. For the purpose of increasing the concentration of a pigment in a toner, the amount of a synergist to be added needs to be increased in accordance with the increase in the concentration of the pigment, and thus the effect of the synergist on the chargeability of the toner increases.
Unexamined Patent Publication No. 2000-338722 proposes a toner excellent in chroma and color developability. This toner has high transparency and can be prepared by highly dispersing a pigment in a binding resin with a high shearing force. However, since a slight amount of an unreacted pigment is present in the toner, the toner is not suitable for the demand for enhancing toner transparency, especially when the toner contains the pigment in an increased concentration.
Organic pigments for use in toners for electrostatic image development contain in their structure a chromophore, which mainly determines the absorption wavelength range of the pigments, and an auxochrome, which by itself does not produce a color but slightly changes the hue of the pigments. Unreacted products and by-products produced as impurities at manufacture of organic pigments have part of the structure of the organic pigments, which are a final product, but they lack part of the chromophore and part of the auxochrome of the final product. Further, organic pigments, which are designed to have a great molecular weight in order to acquire weather resistance, have many ring structures such as a benzene ring, and thus, are substantially an impurity. Many of organic pigments containing these ring structures have absorption wavelengths in the ultraviolet region. Impurities, which have part of the chromophore and part of the auxochrome, have absorption wavelengths shifted in the visible radiation region, and thus, in many cases, their maximum absorption wavelength differs from that of organic pigments, which are a final product. In other words, when a toner contains impurities, the absorption wavelength range of an organic pigment as a whole contained in the toner is widened to darken the toner and reduce its transparency. This is not a significant problem with conventional toners containing an organic pigment typically in a concentration of about 5 wt %, but has a serious effect when the concentration of an organic pigment exceeds about 8 wt % in order for the amount of a toner consumed to be reduced.
In order to solve the above prior art problem and meet the demand for an improvement in image quality and a reduction in the amount of a toner consumed, the present invention provides a toner for electrostatic image development, which while containing an organic pigment in an increased concentration, has a high chroma and a high transparency, and a process for preparing the same.
The present inventors have found, as a result of eager study for solution of the above problem, that a toner for electrostatic image development which, while containing a pigment in an increased concentration, is excellent in chroma and transparency, can be obtained when the toner satisfies particular conditions on the concentration of the organic pigment contained therein and on the dispersion particle diameter of the pigment (particle diameter of the pigment when the pigment is present in the toner).